All natural extermely potent detergent for all things white:
Mix 1/2 white vinegar with 1/2 water in a bucket and soak a few (extraordinary) soap nuts in it. The vinegar will make your horse smell a bit funky, but it not only cleans amazingly well, it helps keep the flies off too.
Simply sponge it on to a dry horse and scub him down then give him a good rinse. You can use this on just about anything white with no worries of harmful toxins or chemicals!